Solid state imaging devices typically include a photosensor such as a photodiode formed in or on a substrate, a color filter formed over the photosensitive device and a microlens array formed over the color filter. The photosensor may be a photodiode, a CMOS (complimentary metal oxide semiconductor) sensor or a charge-coupled device (CCD), for example. N+ type photosensors (NPS's) are commonly used. Between the photosensor and the color filter there is generally a relatively thick inter-metal dielectric (IMD) that accommodates multiple levels of metallization used as an interconnection medium in the peripheral circuits of the solid state imaging device. It is clearly desirable to maximize the sensitivity of the imaging device, i.e., the amount of light that reaches the photosensors. One shortcoming of imaging devices is that the sensitivity of the photosensor is proportional to the fill factor which is defined as the ratio of the area of the photosensor to that of the pixel area. The thick inter-metal dielectric makes it difficult to align the lens and photosensor and insure that the light which passes through the lens, reaches the photosensor. The amount of light that is not directed to the photosensor increases with increasing thickness of the IMD. Furthermore, the interconnect metal disposed within the inter-metal dielectric may reflect incident light further reducing the optical signal and therefore the sensitivity of the photosensor.
It would therefore be desirable to produce an imaging device with a modified structure that increases the sensitivity of the imaging device by allowing more light to be received by the photosensors.